1. Field of the Invention
This invention relates to the field of semiconductor devices, and more specifically, to a process designed to form a planar epitaxial cobalt silicide (CoSi.sub.2) suitable for vertical integration of microelectronic devices and for enabling the processing of devices with shallow junctions.
2. Prior Art
In the manufacture of semiconductor devices, the gate length of transistors must be scaled down for each new generation of integrated circuits (IC) in order to achieve higher speed and higher integration density. The source/drain junction depth of transistors must also be scaled down along with the gate length in order to reduce short channel effects. Currently, in a 0.41.mu. device, the source/drain junction depth must be less than angstroms for both NMOS and PMOS. According to constant field scaling, the source/drain junction depth for 0.25.mu. and 0.15.mu. devices will probably be less than 1250 and 750 angstroms, respectively.
As the source/drain junction depth decreases, the source/drain resistance increases significantly. This requires that a self-aligned silicide (salicide) process be used in order to reduce the source/drain resistance as well as gate resistance. In a salicide process a metal is deposited over an MOS structure, and reacts with the exposed silicon and polysilicon to form a silicide. The unreacted metal is then removed from the MOS structure by using a selective etch. In typical processes, the selective etch leaves the silicide over the source/drain regions and on the gates. Since the silicide remains in the desired regions without using a masking step, the process is thus self-aligned. One current process is the titanium (Ti) salicide process. However, as the source/drain junction decreases to below 2000 angstroms and eventually to 750 angstroms, the titanium silicide (TiSi.sub.2) process can run into serious problems. Because the silicide thickness may be only several hundred angstroms in an ultra:shallow junction, the etch selectivity of TiSi.sub.2 to borophosphosilicate glass (BPSG) may not be high enough for the TiSi.sub.2 source/drain to withstand the contact etch. Another problem is that the titanium atoms form compounds with boron (B). This may make PMOS contact resistance very high. It is also very difficult to form shallow source/drain junctions by driving dopant out of TiSi.sub.2. Dopants and TiSi.sub.2 react to form compounds due to a strong chemical affinity between titanium and dopants. For example, the strong chemical affinity between titanium and boron forms the compound TiB.sub.2 resulting in the depletion of the boron dopants. A similar reaction occurs between titanium and arsenic. All this makes it very difficult to achieve ultra-shallow junctions with TiSi.sub.2.
Cobalt silicide (CoSi.sub.2) is a very promising material for ultra-shallow junctions in future generation processes. The CoSi.sub.2 has excellent etch selectivity to BPSG. Cobalt (Co) atoms do not form tightly bonded compounds with arsenic (As) and boron (B) atoms. Thus, CoSi.sub.2 can be used as a doping source to achieve shallow junctions. Wei et. al., U.S. Pat. No. 5,047,367 assigned to Intel Corporation, demonstrates the formation of an epitaxial quality titanium nitride/cobalt silicide (TiN/CoSi.sub.2) bilayer for use in salicide technology.
What is needed is a manufacturable method of forming ultra-shallow junctions compatible with the salicide technology described above. It is also preferred that any such method include the formation of a planar silicide layer suitable for vertical integration of microelectronic devices.